This invention relates to a process for recovering base from salt solutions containing excess acid or base. More particularly, the invention is directed to the recovery of bases such as sodium hydroxide from spent process material, such as spent liquor from a process to produce nickel cadmium batteries. The spent liquor comprises free base and soluble salt in an aqueous solution.
U.S. Pat. No. 4,608,141 discloses a multi chamber two compartment electrodialytic water splitter and a method for using the same for the basification of aqueous soluble salts. U.S. Pat. No. 4,536,269 discloses a multi chamber two compartment electrodialytic water splitter and a method for using the same for the acidification of aqueous soluble salts. These two patents review the use of two compartment electrodialytic water splitters to treat salts.
Reference is made to Principles of Desalination, Shaffer, et al., Chapter 6, Electrodialysis pp. 199-289, Academic Press, 1966, for a review of electrodialysis. Electrodialytic water-splitting in two compartment cells is well known. For example, U.S. Pat. No. 4,391,680 discloses the generation of strongly acidified sodium chloride and aqueous sodium hydroxide by two compartment water-splitting of aqueous sodium chloride. Three compartment electrodialytic water splitters are known in the art. These are disclosed to be comprised of alternating bipolar, anion and cation permselective membranes thereby forming alternating acid, salt and base compartments. U.S. Pat. No. 4,474,771 discloses three compartment electrodialytic water splitters. U.S. Pat. No. 4,740,281 discloses the recovery of acids from materials comprising acid and salt using an electrodialysis apparatus to separate a portion of the acid followed by the use of an electrodialytic three compartment water splitter to regenerate the acid from the salt.
The staging of two conventional two compartment electrodialytic water splitters whereby the base solution is withdrawn from the base compartment of one two compartment water splitter and is fed through the base compartment of the second two compartment water splitter is known. In an attempt to increase the efficiency of bipolar membranes, U.S. Pat. No. 3,111,472 (Oda, et al.) discloses disposing a microporous water permeable cation or neutral membrane in the acid and/or base compartments of the three compartment electrodialytic water splitter.
Although bipolar membranes have been known to be useful for the process of electrodialytic water splitting to generate an acid and a base for many years (Oda et al. U.S. Pat. No. 2,829,095, Chlanda et al. U.S. Pat. No. 3,787,304, Jenczewski et al. U.S. Pat. No. 4,552,635) and their use in various cell configurations has been reported (Oda et al. Japan. 2023 ('58) reported in Chemical Abstracts 53:11070b. U.S. Pat. No. 4 536 269 and U.S. Pat. No. 4 608 141) their use as highly selective membranes which effectively transport only H+ or OH- ions therefore making them effective for the purification of acids or bases without the generation of the opposite product has not been realized.
None of the above references disclose separating a base from salt containing solution using a bipolar membrane, in an apparatus such as a two compartment water splitter, followed by removing the salt solution.
The art, teaches that anion membranes preferentially permit the selective transport of hydroxal ions over other anions such as Cl.sup.-, SO.sub.4.sup.=, nitrate, or fluoride, but the selectivity is small.